[Feature Request]: Add pthread support wasm build

[mcbarton]

Description of new feature

Emscripten builds can be built with pthreads support (see https://emscripten.org/docs/porting/pthreads.html). I've tested locally and it is possible to build CppInterOps with pthread support, but given its limited browser support (as far as I can tell from the link it only currently works on very recent versions of Firefox, but don't know how often the documentation is updated). It may not be worth putting in for now. Leaving this issue as a potential future improvement once pthreads is better supported by more browsers.

[mcbarton]

@anutosh491 can you comment on how useful you think pthreads support would be for the wasm build? It is currently experimental (but what isn't when it comes to emscripten), but maybe we can make use of multithreading to get better performance for xeus-cpp.